This invention relates to magnetic tachometers and particularly to an improved tachometer for monitoring the speed of a rotary member which forms part of rotating equipment such as a motor.
One example of the use of the present invention is in a video tape recorder, in which tape must be driven at a controlled speed past a scanner assembly which normally comprises a drum around which tape is wrapped in a helical path so as to be scanned obliquely by a head which is driven at a high, controlled speed. In order to control the speed of, for example, the capstan within close limits, it is usual to provide a tachometer generator in order to derive a feedback signal for a servo-mechanism for the capstan. This is particularly important in order to ensure stability of the recording or playback of a video signal on magnetic tape.
The present invention finds particular application in a battery operated portable video tape recorder which requires a capstan and motor of very low inertia. The design of a tachometer for such and similar purposes presents considerable problems, for it is desirable to achieve a tachometer which can produce a high frequency output yet be of comparatively simple and lightweight construction.
It is therefore one object of the present invention to provide an improved tachometer.
Although the present invention will be described in relation to a tape-driving capstan, the tachometer described herein is also of use elsewhere in tape transport, for example in a scanning drum of a helically scanning video tape recorder. It is equally desirable to provide control of the speed of the scanning drum and for this purpose to provide a tachometer which can conveniently be incorporated between the rotary and fixed parts of such a drum.